Recent reports suggest that platelet activating factor-acetylhydrolase (PAF-AH) and paroxonase (PON), two enzymes transported on HDL, play important roles in preventing oxidation of LDL and in so doing protect the artery wall from oxidative events that trigger atherosclerosis. A major hypothesis in the present proposal is that PAF-AH plays an integral role in protection of the vascular wall from oxidative damage. This hypothesis will be addressed in mice in which the PAF-AH gene has been deleted; knockout mice will be bred into the atherosclerosis-susceptible stains. It is predicted that such mich will show exacerbated lesion formation that correlates with increased plasma hydroperoxiedes and MDA-lysine titer. Conversely, the PAF- AH gene will be expressed in increasing doses in transgenic mice and aortic lesion formation and lipid hydropeoxides will be quantified and correlated with PAF-AH activity and mass; it is predicated that lesion formation will be reduced in a dose dependent manner. It is suggested that PAF-AH's ability to hydrolyze oxidized phospholipids is highly dependent on the presence of specific apoB100 subpopulations; therefore, human PAF-AH will be overexpressed in atherosclerosis prone transgenic mice expressing human apoB100. This model will be useful for examining the determinants for PAF-AH association with a specific LDL subpopulation(s), thus elucidating the role of minor proteins and/or lipid factors in PAF-AH reactivity and protection of LDL from oxidation. Macrophages are the sole source of PAF-AH; therefore, macrophage cultures will be used to elucidate the initial steps in the biogenesis of PAF-AH-HDL and to test whether apoAI, apoAII, HDL and specific LDL subclasses regulate synthesis and/or secretion of PAF-AH. The enzyme, PON, is transported primarily on apoAI without apoAII HDL. Putatively, PON-HDL protect LDL from oxidative stress. This proposal will address the question whether overexpression of mouse PON in atherosclerosis-susceptible mice protects such animals from diet-induced lesions and whether protection is associated with the increased potential of the PON-HDL to protect LDL from oxidative events. The project will elucidate the early steps in PON-HDL biogenesis by studying the assembly of nascent apoAI- PON particles. The proposed studies will provide new insights into the physiological role of PAF-AH and PON in the atherosclerotic process and will help in the development of therapies to prevent early atherosclerosis.